Government seeks to avoid disrupting flow of people and goods between Ireland and Northern Ireland

LONDON—The U.K. will on Wednesday rule out extending the European Union’s customs area to encircle Northern Ireland after Brexit, saying such a move wouldn’t be “constitutionally or economically viable.”

In a formal position paper setting out London’s principles for resolving the Irish border issue in negotiations with Brussels, the U.K. will say it doesn’t want customs posts or other checks to disrupt the flow of people and goods...